The Truth about Tone Capacitors

(C) 2010 Hank Wallace & Chad Barbour

Featured Products

Do an Internet search for "Tone Capacitor" and you will find the
most amazing mish-mash of fact, half-fact, lies and opinions. This
type of capacitor is better than that one. This dielectric is
brittle, and that one sounds smooth. This cap is better for rock and
that one better for country. This one is more Fender-ish, and that
one more boutique. Another hypefest just begging to be busted!

That's what we love: Busting hypefests! So let's get to it.

Briefly, a capacitor (in the context of musical electronics)
consists of two plates of conducting metal separated by an insulating
layer (called a dielectric). For example, if you took two
rolls of aluminum foil and unrolled them, sandwiching a layer of
paper between, that would make a capacitor. To make it more space
efficient, you would roll the whole thing up again. If you connected
a wire to each roll of foil, you would find that the two insulated
conductors have some interesting properties.

One of those properties is that a capacitor increases or reduces
high frequency audio energy, depending on how it is connected inside
a guitar or amplifier. Now you won't be able to fit your handmade
capacitor inside your guitar, but there are mass produced capacitors
that fit nicely, and change the tone of your instrument by rolling
off the high frequencies, in conjunction with the guitar's tone pot.

A capacitor has a value called the capacitance, measured in
Farads (after a long dead European scientist), but the Farad turns
out to be a HUGE unit of measure. Engineers typically measure
capacitance in units a million times smaller, called microfarads.

A common value for a tone capacitor in guitars is 0.022
microfarads, or 0.022uF as an abbreviation. A larger value that rolls
off high frequencies to a greater degree is 0.1uF. There are many
values used in the wide range of guitars on the market, but the tests
here focus on these two values. The principles learned apply to other
values as well.

Then how do we bust this hypefest? We do that by attaching tone
capacitors of the same measured value to the same
instrument, in turn, and measuring the frequency response of the
pickups. For a guitarist that's a difficult task as the only
measurement tool he has is his ear, and that tool is biased by all
the other stuff running around in his brain, good and bad.

Fortunately, we have a tool that we created specifically for
measuring the frequency responses of guitar pickups, and it has no
such biases. Using that tool, we can measure the responses, plot
them, and even create sound clips so that we can actually hear
the effects of various tone capacitors.

We first purchased a load of capacitors of various types, all new.
These represent a wide range of capacitor types used in guitars and
referenced in the online hypefest. This was not cheap, but truth is
sometimes costly!

We also scrounged our personal inventories of surplus parts for
capacitors of the two values we were testing. One associate who is an
electronics junkie from way back (just ask his wife) found a couple
of the coveted and ridiculously expensive Sprague Vitamin Q paper in
oil capacitors, so we grabbed those for testing as well.

We performed two tests:

We selected a number of capacitors of nearly the same value but with
different dielectrics, then tested and recorded the frequency response of
a real pickup with each capacitor connected in turn. This showed
us the effect of the dielectric type and construction on the behavior
of the capacitor.

We selected a number of capacitors of the same marked value, but with
actual values that vary, to see what the range of frequency response
variations is when the user simply assumes that the marked value is
the actual value.

We used a Fender Stratocaster with a single coil bridge position
pickup to perform these tests. We selected a single coil pickup
because the lower interwinding capacitance and inductance of the
pickup allows tone capacitor value variations to affect the frequency
response to a greater degree, making it easier to understand and view
the changes. We disconnected the tone pot and capacitor inside the
guitar before performing these tests, to avoid those as tone
determining factors.

We measured 90 capacitors to find ones close and far apart in
value, of many different types. We tested 26 of those in the
instrument.

The tabulated results appear below. Click the pictures to see a
larger capacitor image. The sound clips were created as on our pickup
test page, by filtering a stock sound clip
using the curves you see below. This is MUCH more accurate for
comparisons than strumming a guitar due to the variations in playing
and player digestion.

(You may have to zoom the graphs to read the legends as there are
multiple curves on each graph. The actual value of each capacitor is
shown in microfarads on the legend for each curve.)

Analysis and Conclusions

The data and sound clips above show clearly that for tone
capacitors of close measured capacitance value there is no difference
in tone. For example, compare the clips and plots for the "Monolithic
X7R 0.022uF - 0.0204" and the "Vishay 225P Orange Drop 0.022uF -
0.0204". (You can click on the colored box in the legend and the line
will be highlighed briefly.) These two capacitors, of the same
measured value (0.0204uF) but different dielectrics have
indistinguishable plot lines, differing in frequency at the 0dB
crossing by about 0.3%. This difference is below the rated accuracy
of our measurement equipment and is statistically insignificant.

The capacitor values in the 0.022uF graph vary from the 0.022uF
nominal value -15.5% to +12.7%. (The obvious outlier, the ceramic
disc, was a junk drawer special from a Radio Shack grab bag.) The
range of frequencies where the curves cross 0dB headed down on the
right is about 728Hz (-11%) to 906Hz (+11%), using linear
interpolation. The actual dependency is nonlinear, but we used linear
interpolation as an approximation to compute these percentages.
Going further and plotting a frequency vs. capacitance curve in a
spreadsheet and fitting a quadratic polynomial, the correlation
coefficient is better than 0.97.

We found only moderate statistical corrleation between the voltage
rating of a capacitor and the frequency response (0.45). However, the
correlation between the value and voltage rating of all 90 capacitors
was 0.40, explaining the former correlation and eliminating voltage
rating as a tone determining factor.

It appears that the variation in frequency response is due to the
values of the capacitors and not any other factor. From the sound clips,
you will notice that you can hear NO difference between the
capacitors, except for the most extreme tolerance variations.

There is no advantage to using large, high voltage capacitors. The
smaller, lower voltage units work as well.

This dependency on capacitance value alone is EXACTLY what any
trained electrical engineer would expect. EE's select capacitors
based on several properties, including the tolerance and dielectric
type. For example, some radio circuits require tight tolerance
capacitors, 5% or even 1%. Many radio circuits require capacitors
that do not vary much over temperature, and capacitors with "NP0" or
"COG" dielectrics fit that requirement. Ceramic capacitors are in
the main microphonic, but polyester caps are not. Polycarbonate
capacitors are stable for use in audio oscillators. Ceramic
capacitors have low resistive leakage and are stable over time. There
are many factors, but in a guitar application, just about any modern
capacitor works well.

To be totally anal, we should present a detailed statistical
analysis of the data. But that's not very useful since facts
(statistical or otherwise) generally don't have an impact on
hypefests anyway. The curves and sound clips tell the story very
well.

However, there are a thousand YouTube videos that show well
meaning guitarists soldering various tone capacitors into their
guitars and raving about the tone. They rave about the improved tone
related to factors like the type of dielectric in a capacitor, where
this is clearly not true. What's up?

Here's what could be happening:

The guitarists are simply falling prey to a musical community
hypefest.

The guitarists want to believe that a certain capacitor
has a better sound, so they believe it.

The capacitors they are testing have different values due to wide
component tolerances. Every capacitor has a tolerance as
manufactured, typically +/-10% or +/-5%. Older capacitors can drift
greatly with time. We tested one capacitor (not in the table) whose
value drifted continuously during measurement. It had a different
sound, because it was DEFECTIVE.

When you see a video or read an article regarding tone capacitors,
if the capacitors being compared are not being verified and measured
using a capacitance meter, then the entire exercise is a waste of
time. Each capacitor, especially the old ones, must be verified to
be within tolerance or tone comparisons are useless.

There is resistive leakage occurring within the capacitors.
This has the effect of sounding like a resistor has been added in
parallel with the capacitor. Pickup responses are sensitive to such
resistances, and older capacitors can skew response measurements and
sounds if they are leaky.

We found that only one capacitor of the old ones we tested was leaky. We
discarded it as defective.

This leads us to the NOS (new old stock) phenomenon. The operative
word in this phrase is OLD. Would you buy new old stock food? No?
Why? BECAUSE IT IS OLD. If you buy NOS capacitors that have been
sitting in a box since 1950, don't expect them to be in tolerance or
have low leakage. The designers of the capacitors did not expect them
to be used 60 years after the manufacturing date!

If you want to get the sound of a 60 year old capacitor that
should have been trashed 50 years ago, just buy a $0.50 modern
capacitor and place a one-cent one-megohm resistor in parallel with
it. There you go. I just saved you $39.49.

To answer critics who would say, "But you tested the capacitors
with no tone pot in the circuit, man. When I turn the tone pot down
to 5, man, that's where I hear the difference. Man." If there's no
difference between capacitors A and B with no tone pot, there will be
no difference with the tone pot connected, at any setting. Man.

What about the super expensive silver plated capacitors? Are they
better? One would think that if such parts were great in audio
amplifiers, the would also be great in cell phones, radar systems and
stealth bombers. But they are not used anywhere but in the boutique
audiophile and guitar markets. That is, electrical engineers are not
susceptible to marketing hype and forum phlegm! We have a
related article on gold plated connectors and oxygen free copper that
you should read, What About Gold Connectors and
Oxygen Free Copper?

Our conclusion is that two tone capacitors with the same measured
capacitance value and low leakage yield the same tone, regardless of
rated voltage, size, dielectric, or price!

Hypefest busted! Have a nice day.

What about Capacitors in Amplifiers?

An astute reader has asked about the effect of various types of
capacitors in tube amplifiers. Good question.

The same conclusion above applies. As long as the capacitor is
rated for the proper voltage, two capacitors of the same measured
value will sound the same. Proper installation must of course be
observed for polarized (electrolytic) capacitors.

There are some other factors that bear mention. By far the
leakiest capacitors in a tube amplifier are the electrolytic power
supply filter caps. These drop in value and get leakier with age, and
their internal resistance increases. If you are building an amplifier
as a copy of an amp whose sound you love, the condition of the filter
capacitors is important. Old, drooping value, leaky capacitors make
for a poorly regulated power supply, and that will definitely affect
the tone, especially the response to transient, fast signals. The
brand new capacitors in the copy of the amplifier will make the
amplifier sound different in subtle ways.

Now I would consider the older capacitors defective, but
you might like the sound they produce. You could get that sound by
selecting the value properly. A lower capacitance value will produce
a different tone, but will also produce more hum from the amplifier.

Another factor to consider in amplifier design is the microphonic
nature of each part. Some capacitors (notably, ceramic) pick up sound
like a small microphone. This can be a huge problem if your amp is
mounted in the same air space as the speakers it drives. This creates
a direct feedback path from the speaker into the amplifier circuit, so
when you turn the amplifier up, you hear a squeal.

You can tell if a capacitor is microphonic by tapping on it with a
plastic implement while the amplifier is running, and listening to
the speaker for corresponding sounds. (Please observe all safety
precautions when doing this. We will not take responsibility for your
electrocution.)

If you are copying an amplifier, it would be best to actually
measure the value of each component in the circuit. As I noted in the
article, What's with the Tube Hype? some
components in old tube amps can be expected to drift 50% or more with
age. To get the same tone, you have to use the same component values,
which may be totally out of spec for the tubes and supply voltages
you are using. You should analyze the circuit to ensure safe
operation at all times, under all operating conditions.

You might be a stickler for using original parts in your
amplifier, and that's fine if you want something that looks like the
real thing from the 1970's. However, you are going to pay more for
the parts and you might sacrifice longevity. Why use carbon
composition resistors when today's standard is carbon or metal film?
I have replaced a lot of out-of-value carbon comp resistors in
amplifiers over the years, and you will, too! Why seek out old paper
in oil capacitors when a good mylar unit will work as well and last
forever? It's your choice.

Electrical engineers do select components based on their
construction, but mostly based on their value, tolerance, temperature
characteristics, and price (all related to construction). And when you
are tweaking component values in search of that killer tone, know
that it is the measured value of the component that affects
the tone to the greatest degree, and not other factors.

Disclaimer: I'm assuming here that you know what you are doing in
selecting components for use in high voltage tube amplifiers. If you
select a capacitor that is under rated for the voltage in the circuit
(plus allowance for signal peaks, and AC line variations and spikes),
you are in for some dangerous gigging. A capacitor generally reacts
to an overvoltage condition by exploding, so you have to know
what you are doing. Have a proper electrical engineer review your
work before you flip on the power switch. Always obey good design
practice. We are not responsible for your design errors or injuries
you cause.

Patent This

Now that the heavy lifting is over, let's have some fun. Similar to
the idea in our article about gold connectors and oxygen free copper,
imagine if you could dynamically change the dielectric in a
capacitor! Take a look at this baby:

Just think of it. Connect an aquarium pump to a hydraulic foot
switch, and have several tubes running to various dielectric
solutions backstage. Want a more organic musical sound? Pump in some
all natural mineral oil. Playing a synth heavy dance tune from
the 1980's? Open the synthetic motor oil valve for that
plastic sound. And for the Birkenstock and sprouts crowd, you
would of course have a tank of happy sunflower oil in reserve.

There you go (again). The variable dielectric capacitor. Race you
to the patent office?

P.S.

To head off another subsidiary hypefest that pops up periodically,
some say that tone capacitors are directional. That is, they have two
wires and the two different ways of connecting them results in tonal
differences. Now some capacitors are directional. The
term is polarized. For example, the electrolytic capacitors
used in the power supply of your tube amp are polarized. Connect one
of those backwards and you will have a mess on your hands.

But for tone caps of the type we tested, there are no polarity
issues. The capacitors function and sound the same whichever way
they are connected. Any experience to the contrary exposes a
defective capacitor.

What about the 'outside foil' consideration? When a capacitor is
wound of foil, naturally one side of the metal sandwich is on the
outside of the finished part. It has been noted that this outside
foil can pick up noise and change the sound of a capacitor. This is
simply and indication that the rest of your circuit is poorly
shielded! Inside the metal chassis of a tube amplifier, this
factor is immaterial, since the chassis provides shielding. In a
guitar, this might be a problem. However, I have shielded the
cavities of all my instruments with copper foil, haven't you?

Here we see that a 'capacitor tone factor' is actually a leftover
of poor design on the part of instrument manufacturers. Buy
some foil tape at Home Depot and fix that instrument!